Desulfurizing of steel



DESULFURIZING OF STEEL Werner Coupette and Adolf Sickbert, Wattenscheid- Eppendorf, Germany, assignors, by mesne assignments, to Bochumer Verein fiir Gussstahlfabrikation Aktiengesellsclhaft, Bochum, Germany No Drawing. Application January 10, 1956 Serial No. 558,196

Claims priority, application Germany January 13, 1955 12 Claims. (Cl. 75-49) The present invention relates to the desulfurization of steel, and more particularly to a new method whereby steel may be extensively desulfurized in a single extremely rapid proceeding.

For most practical applications of steel, any sulfur content in the steel is deemed deleterious and it is therefore an extremely important task to provide for elimination of the sulfur, or at least as much sulfur as possible, during the steel smelting processes.

However, the known desulfurization methods which are utilized in connection with steel smelting require prolonged periods of treatment and are of relatively low efficiency.

It is therefore a primary object of the present invention to provide a new method of desulfurizing steel whereby the time of treatment is considerably shortened as compared to known methods and further whereby a high degree efliciency with respect to removal of the sulfur is achieved.

It is another object of the present invention to provide a novel method of effectively desulfurizing steel to particularly low sulfur content according to which method there is a high degree of certainty and efiiciency in the desulfurization.

Other objects and advantages of the present invention will be apparent from a further reading of the specification and of the appended claims.

With the above objects in view, the present invention mainly comprises the desulfurization of molten steel by pouring the molten steel (which may be killed, partially killed or not at all killed) through a partial vacuum into contact with a desulfurizing agent such as a desulfurizing slag or slag-forming mixture.

According to the preferred embodiment of the present invention, the molten steel is poured through the vacuum, that is into a chamber which has been evacuated of gas, under such conditions which will be more fully described later that the stream of molten steel as it enters the vacuum chamber becomes subdivided into fine droplets.

These fine droplets then come in contact with and react with the desulfurizing agents in the vacuum chamber to result in an extremely effective and rapid desulfurization of the steel.

The basic method of the presentinvention of contacting the molten steel with the desulfurizing agents under vacuum conditions by pouring the molten steel into a vacuum chamber into contact with the desulfurizing agents is preferably carried out utilizing a pressure below 500 mm. Hg pressure in the vacuum chamber. The desulfurizing agents which are utilized should generally be basic, and if necessary these agents may be such as to act also as reducing agents.

It is further advisable according to the present invention to make certain that the content of heavy metal oxides such as iron oxide is maintained at a low level. Liquid slags are preferred for utilization as desulfurizing agents, however finely divided solid slags may also be added, and if necessary these may be preheated.

2,848,317 Qfi Patented Aug. 19, 1958 It may be seen in this connection that primarily synthetic slags are most suitable for the desulfurization because the same can be prepared in exact and constant composition and further because they are free of impurities such as heavy metal oxides. It is also possible to utilize slag-forming materials in combination with finished slags, and particularly where it is desired to operate without finished slags it is possible and desirable to utilize mixtures of slag-forming materials.

Any common desulfurizing agents may be utilized according to the present invention. The present invention is not mainly concerned with the use of any particular desulfurization agent, but rather in the manner in which the molten steel to be desulfurized is contacted with the desulfurizing agents to achieve maximum and most efiicient desulfurization.

The desulfurizing agents may be introduced into the vacuum chamber prior to the pouring of the molten steel into the same. However, it is preferred to introduce the desulfurizing agents into the vacuum chamber simultaneously with the molten steel, whereby the desulfurizing agents may be introduced at once as one complete batch, in portions, or continuously during the introduction of the molten steel.- The desulfurizing agents may be separately introduced into the vacuum chamber through an air lock arrangement, or they may be added to the molten steel prior to introduction of the same into the vacuum chamber.

It is possible in accordance with the persent invention, and particularly by the introduction of the desulfurizing agent into the vacuum chamber simultaneously with the pouring of the molten steel into the same, to desulfurize in a continuous process whereby desulfurized steel and slag are removed from the desulfurizing chamber either intermittently or continuously. In this manner it is possible without difficulty to draw-01f steel at or near the bottom of the vacuum chamber and to withdraw slag by means of at least one opening at a predetermined distance from the bottom of the vacuum chamber. The opening for the withdrawal of the slag. should preferably be kept under vacuum conditions by means of a shunt arrangement.

As indicated above, according to the preferred embodiment of the present invention the stream of molten steel is passed into the vacuum chamber under such conditions of degree of vacuum, quantity of molten steel flowing into the vacuum chamber per time period, the ferrostatic pressure of the column of molten steel above the opening through which the stream of steel is passed, and the size of the opening through which the molten steel is passed that the jet of molten steel while falling freely through the vacuum chamber disintegrates into a plurality of individual small droplets of molten steel. hese droplets then contact in the vacuum chamber the desulfurizing agents whereby a rapid and thorough contact between the molten steel droplets and desulfurizing agents is achieved which results in a rapid and highly effective desulfurization of the steel.

It is apparent from the above that the disintegration of the stream of molten steel when exposed to the subatmospheric pressure is achieved by the interrelationship and correlation of the several factors set forth above. The particular value for any particular condition will depend upon the correlation of that value with the values for the other conditions. However, anyone skilled in the art will be able to determine any particular value by simple pretesting, particularly in view ofthe following disclosure.

The opening through which the stream of molten steel enters the vacuum chamber (the pan nozzle) should be so chosen as to have a diameter of between about 25 and 70 mm. The corresponding vacuum should be within the range of between about 30 and 0.1 mm. Hg of pressure. If the steel to be desulfurized is the type which customarily possesses a particularly high gas content, such as in the case of only partially killed or not at all killed steel, the degree of vacuum should be chosen at within the upper portion of the above range (that is at a higher pressure or lower degree of vacuum) while in the case of utilizing killed steel the degree of vacuum should be at the lower portion of the range (that is at a lower pressure or higher degree of vacuum).

Molten steel which has a high gas content, i. e. gasrich steel, should be poured from nozzles. of larger diameter, while gas-poor steel should be poured from nozzles having smaller diameters. Furthermore, it is advisable to provide a ferrostatic pressure which will result from the use of a molten steel column at least 300 mm. high above the opening of the nozzle into the vacuum chamber.

The proper control of the above factors within the indicated ranges results in the disintegration of the jet or stream of molten steel in the vacuum chamber into droplets having a diameter of between about 10 and 10- mm., within which range the best results are achieved with respect to the desulfurization of the steel by the contact and reaction of the droplets of steel with the desulfurizing agents.

The proper determination and choice of the various factors set forth above can be controlled by taking a great number of photographs very rapidly within a short time period (several thousand exposures per second) so that it is possible to measure on the photographic images the absolute size of the droplets.

According to the present invention it has furthermore been found that the steel in this finely divided dispersion is especially well suited to react efiectively and quickly with the slag. The thus achieved efiect considerably surpasses the results which are obtained by pouring the steel into the vacuum chamber and bringing the molten steel jet or stream in contact with desulfurization agents without disintegrating the steel jet into droplets in accordance with the preferred embodiment of the present invention.

The disintegration of the molten steel stream into small droplets is of particular advantage in combination with the reduced pressure in the vacuum chamber which furthers the ability of the steel to give-off gas during the desulfurizing reactions. The disintegration of the molten steel into small droplets enlarges the reacting surface of the steel, and the giving-off of the gas by the steel further enlarges the reacting surface thereof. It is therefore ad vantageous according to the present invention to utilize r steel which has a high gas content.

It is also within the scope of the present invention to so modify the steel smelting process which precedes the reduced pressure desulfurizing treatment, such as the open hearth, Thomas, Bessemer, top blast smelting process (with air, oxygen-enriched air, pure oxygen or any other suitable refining gas), electro-smelting or other refining process, that, by combining the refining process with the desulfurizing process of the present invention, a simplification or improvement of the production is achieved for instance by catching the slag, or in such a way that less care need be taken with respect to the choice of the starting materials such as pig iron, scrap and additions. It is thereby immaterial whether or not the steel contains relatively large quantities of oxygen, nitrogen or hydrogen. In the case of oxygen-containing steel it is advantageous to provide, prior to the vacuum desulfurization treatment, for at least as much carbon as is necessary to create in not completely killed steels under vacuum a vigorous C formation.

In accordance with the present invention the slag should be capable of reacting in the same way during the entire reaction period in the vacuum chamber. This is. achieved. by the addition of material required for a continuation of the reaction, or by increasing the amount of slag during the desulfurizing treatment. It is additionally significant that the slag in the vacuum chamber swells up in a foam-like manner so that the reaction space and reaction time are increased. It is therefore advantageous to utilize for the desulfurizing reaction in accordance with the present invention slag which has an ample gas content.

Apart from the normal advantages of continuous operation, continuous execution of the desulfurization process according to the present invention has the further advantage of reducing the losses of heat and temperature. Continuous desulfurization in a vacuum desulfurizing chamber according to the present invention is especially advantageous for steel smelters in which frequent tappings follow each other, where the smelting process is of short duration and where large quantities of steel are produced, as is the case with all kinds of blowing processes. The present invention however is not limited to steel obtained by any specific smelting process.

In view of the heat loss experienced during the pouring of steel into the vacuum desulfurization chamber, it is desirable to increase the temperature of the molten steel prior to enter-ing the vacuum chamber.

It is also desirable to reduce the carbon or silicon content of steel entering the desulfurizing chamber in all such cases where during the reaction in the vacuum chamber an increase in the carbon or silicon content of the steel occurs. When working under strongly reducing conditions, it is possible to add to the slag materials (for instance compounds) which during the treatment in the vacuum chamber will introduce into the steel desirable metals such as molybdenum, tungsten, vanadium, boron, tantalum, niobium or the like. It is also within the scope of the present invention to add to the slag metals or alloys, especially silicon or aluminum for introduction into the steel.

Thus, as has been described above the method of the present invention permits in various modifications to obtain extensive desulfurization of steel in a single, fast and effective manner.

Although the scope of the present invention is not meant to be limited to any specific operational details, the following examples are given to further illustrate the present invention.

Example 1.60 tons of a steel containing 0.40% C, 0.05% Mn, 0.20% Si, 0.021% P and 0.045% S and having a gas content of 8 p. p. in. H 0.006% p. weight 0 and 0.008% N and a temperature of 1670 C. were poured from a ladle gas-tightly arranged upon the cover of a vacuum chamber into a second ladle which was enclosed in the vacuum chamber. The ferrostatic pressure was caused by the fluid steel column of 0.51.5 m. height. The outlet of the upper ladle had a diameter of 2% inches. The beginning of and during the pouring a pressure of about 350 mm. Hg was maintained in the vacuum chamber. During pouring, 2.5 tons of a molten slag containing CaOAl O -SiO +MgO+CaF kg. FeSi (90% Si) and having a temperature of 1640 C. were introduced to the vacuum chamber through .another inlet in the cover in such a way that the slag came into contact with the stream of the molten steel. After a reaction time of 8 minutes 30 seconds, the steel had the following analysis: 0.38% C, 0.48% Mn, 0.29% Si, 0.021% P and 0.028% S and a gas content of: 7 p. p. in. H- 0.005% 0 and 0.008% N The temperature of the treated steel: 1640 C.

Example 2.In a similar way another steel was treated under the following conditions: analysis of the steel untreated: 0.35% C, 0.55% Mn, 0,01% Si, 0.019% P, and 0.048% S. Gas content: 9 p. p. m. H 0.008% 0 and 0.009% N Temperature: 1680 C. Ferrostatic pressure: 0.5-1.7 m. Diameter of the pouring outlet: 2% inches. Pressure in the vacuum chamber: 2 mm. Hg. Quantity of the steel: 60 tons. Quantity of the desulfurizing slag: 2.5 tons. The slag containing CaO-- A1 O SiO +MgO+CaF -l-225 kg. FeSi (90%Si). Temperature of the slag 1650 C. Duration of reaction 9 minutes, seconds. Analysis of the treated steel: 0.32% C, 0.51% Mn, 0.27% Si, 0.019% P and 0.017% S. Gas content of the treated steel: 3 p. p. m. H 0.002% 0 and 0.005% N Temperature of the treated steel: 1645 C.

Example 3 .In a similar way another steel was treated under the following conditions: analysis of the steel untreated: 0.37% C, 0.58% Mn, 0.24% Si, 0.017% P and 0.042% S. Gas content: 6 p. p. m. H 0.004% 0 p. weight and 0.006% N Temperature: 1685 C. Ferrostatic pressure: 0.5-1.5 in. Diameter of the pouring outlet 2% inches. Pressure in the vacuum chamber: 1 mm. Hg. Quantity of the steel: 60 tons. Quantity of the desulfurizing slags: 2.5 tons. The slag containing Si). Temperature of the slag 1660 C. Duration of reaction: 9 minutes. Analysis of the treated steel: 0.36% C, 0.56% Mn, 0.34% Si, 0.017% P and 0.013%

S. Gas content of the treated steel: 2 p. p. m. H

0.001% 0 and 0.004% N Temperature of the treated steel: 1640 C.

Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can by applying current knowledge readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention and, therefore, such adaptations should and are intended to be comprehended within the meaning and range of equivalence of the following claims.

What is claimed as new and desired to be secured by Letters Patent is:

1. In a method of desulfurizing steel, the steps of introducing a stream of molten steel into a vacuum chamber under a pressure of between 30-01 mm. Hg through an opening having a diameter between -70 mm. and a sufficient ferrostatic pressure of the molten steel above said opening through which the same is introduced so that said stream of molten steel is disintegrated into droplets having a size between 10 and 10- mm. upon introduction of said stream into said vacuum chamber; and contacting the thus formed droplets of molten steel in said vacuum chamber with a desulfurizing agent so as to desulfurize said molten steel.

2. In a method of desulfurizing steel the steps of introducing a stream of molten steel into a vacuum chamber under a pressure of between -01 mm. Hg through an opening having a diameter between 25-70 mm. and a sufficient ferrostatic pressure of the molten steel above said opening through which the same is introduced so that said stream of molten steel is disintegrated into droplets having a size between 10 and 10- mm. upon introduction of said stream into said vacuum chamber; and contacting the thus formed droplets of molten steel insaid vacuum chamber with a desulfurizing slag, said slag having a high gas content, so as to desulfurize said molten steel.

3. In a method of desulfurizing steel, the steps of introducing a stream of molten steel into a vacuum chamber under a pressure of between 30-0.1 mm. Hg through an opening having a diameter between 25-70 mm. and a sufiicient ferrostatic pressure of the molten steel above 7 said opening through which the same is introduced so introduction into said vacuum chamber; and introducing liquid slag desulfurizing agent into said vacuum chamber along with said molten steel so as to contact said molten steel in said vacuum chamber with said desulfurizing agent, thereby desulfurizing said molten steel.

4. In a method of desulfurizing steel, the steps of introducing onto a desulfurizing agent in a vacuum chamber a stream of molten steel into said vacuum chamber under a pressure of between 30-01 mm. Hg through an opening having a diameter between 25-70 mm. and

a sutlicient ferrostatic pressure of the molten steel above said opening through which the same is introduced so that said stream of molten steel is disintegrated into droplets having a size between 10 and 10- mm. upon introduction into said vacuum chamber so as to contact the thus formed droplets of molten steel in said vacuum chamber with said desulfurizing agent and thereby desulfurize said molten steel.

5. A method according to claim 1 in which said molten steel is continuously introduced into said vacuum chamber, and desulfurized steel is withdrawn from said vacuum chamber.

6. A method according to claim 1 in which said molten steel also contacts a reducing agent under said vacuum.

7. A method according to claim 1 in-which said molten steel also contacts a reducing agent under said vacuum and in which said molten steel under said vacuum further contacts at least one alloying metal which alloys with said molten steel.

3. In a method of treating molten steel, the step of introducing a stream of molten steel into a vacuum chamber under a pressure of between 30-01 mm. Hg through an opening having a diameter between 25-70 mm. and a suflicient ferrostatic pressure of the molten steel above said opening through which the same is introduced so that said stream of molten steel is disintegrated into droplets having a size between 10 and 10' mm. upon introduction of said stream into said vacuum chamber.

9. A method according to claim 8 in which said molten steel contacts a reducing agent under said vacuum.

10. A method according to claim 8 in which said molten steel contacts at least one alloying metal which alloys with said molten steel under said vacuum.

11. A method according to claim 8 in which said molten steel contacts'under said vacuum a reducing agent and also at least one alloying metal which alloys with said molten steel.

12. In a method of the character described, the step of introducing onto a reducing and desulfurizing agent in a vacuum chamber a stream of molten steel into said vacuum chamber under a pressure of between 30-01 mm. Hg through an opening having a diameter between 25-70 mm. and a sufficient ferrostatic pressure of the molten steel above said opening through which the same is introduced so that said stream of molten steel is disintegrated into droplets having a size between 10 and 10* mm. upon introduction into said vacuum chamber so as to contact the thus formed droplets of molten steel in said vacuum chamber with said desulfurizing and reducing agent and thereby desulfurizing and reducing said molten steel.

References Cited in the file of this patent UNITED STATES PATENTS 1,472,006 Jones Oct. 23, 1923 1,555,313 Rohn Sept. 29, 1925 2,110,067 Heuer Mar. 1, 1938 2,706,152 Derge et al. Apr. 12, 1955 2,750,280 Perrin et a1 June 12, 1956 

1. IN A METHOD OF DESULFURIZING STEEL, THE STEPS OF INTRODUCING A STREAM OF MOLTEN STEEL INTO A VACUUM CHAMBER UNDER A PRESSURE OF BETWEEN 30-0.1 MM. HG THROUGH AN OPENING HAVING A DIAMETER BETWEEN 25-70 MM. AND A SUFFICIENT FERROSTATIC PRESSURE OF THE MOLTEN STEEL ABOVE SAID OPENING THROUGH WHICH THE SAME IS INTRODUCED SO THAT SAID STREAM OF MOLTEN STEEL IS DISINTEGRATED INTO DROPLETS HAVING A SIZE BETWEEN 10 AND 10-3 MM. UPON INTRODUCTION OF SAID STREAM INTO SAID VACUUM CHAMBER, AND CONTACTING THE THUS FORMED DROPLETS OF MOLTEN STEEL IN SAID VACUUM CHAMBER WITH A DESULFURIZING AGENT SO AS TO DESULFURIZE SAID MOLTEN STEE. 